In general, dyes for formation of images need to satisfy the following requirements.
(1) The color images obtained from them need to be fast to heat and light.
(2) The color images need to have a large molecular extinction coefficient and have a good color hue.
In the past, indoaniline dyes obtained from phenolic or naphtholic compounds and p-phenylenediamines have often been used as cyan dyes for forming color images. However, these indoaniline dyes do not always satisfy requirements (1) and (2) above, and further improvement in such dyes has been desired.
Pyrazolotriazoles of azomethine dyes are described in JP-A-64-48862, JP-A-64-48863 and JP-A-63-145281 as improved. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) These dyes do provide a sharp color hue having a small side-absorption in a blue light range, but they are not sufficiently fast to light and heat and are therefore are not completely satisfactory.
On the other hand, cyan dyes having excellent properties for use in a thermal transfer system are substantially unknown.
Indoaniline dyes described in JP-A-64-53893, JP-A-64-20194, JP-A-64-24792, JP-A-64-24795, JP-A-63-290793, JP-A-63-308072, JP-A-63-308072, JP-A-1-176593 and JP-A-1-130990 have relatively good properties. However, these dyes still have a broad absorption and color reproduction in forming images is a problem. In addition, images formed of these dyes still do not have sufficient fastness to heat and light. Dyes having a high extinction coefficient are desired since they easily would provide images having a high density. However, the above-mentioned dyes do not have a sufficiently high extinction coefficient.